A charge pump in a phase compare application is used to output a current in response to a difference between two signals applied to the input of a phase or frequency comparator. A positive or negative phase difference at the input of the comparator is converted to a positive or negative current out of the charge pump. An ideal charge pump used with an ideal phase comparator should produce zero average current if no phase difference exists between the two signals being compared. However a charge pump is never ideal due to DC and transient mismatches inherent to any charge pump topology. Even if positive and negative pump current sources are perfectly matched, the time delay to enable or disable the positive and negative output currents may not be equal. The result of this timing mismatch is that even when signals with no phase difference are applied to an ideal phase comparator driving a non-ideal charge pump, the charge pump output average current will be non zero. The severity of this error worsens as the frequency of the phase comparison increases.
While others have focused on eliminating other sources of charge pump error, such as correcting static current mismatches, there is a need to eliminate these transient residual phase errors to reduce noise and offset control loop errors. The invention described is a general method for reducing the transient mismatches observed in commonly used charge pump architectures. The simple circuit techniques allow for reduced design time by boosting the performance of common architectures so that they may be used in applications demanding higher performance and accuracy.